As fuel costs rise, efforts have been made to improve fuel economy in vehicles. By improving engine efficiency, it may be possible to improve fuel economy. Various systems and methods to improve engine efficiency have been explored.
One such system which may be used to improve energy efficiency is a Rankine cycle system. Rankine cycle systems utilize the Rankine thermodynamic cycle converting heat energy into mechanical work. A Rankine cycle system generally includes the following components: a boiler, a turbine, a condenser and a pump. As an example of a Rankine cycle system, working medium, such as water, may be circulated through a piping system to the various components such that the working medium is expanded to drive a linked mechanical device. For example, water may be supplied by the pump to an evaporator or boiler to generate water vapor. The vapor may be expanded through a turbine generating power output. The vapor may cycle back to a condenser where the vapor may be converted back to the liquid phase.
In the past, attempts have been made to incorporate Rankine cycle systems in vehicles and to improve the efficiency of such systems. For example, U.S. Pat. No. 6,725,662 discloses one approach of using the Rankine cycle for use in a vehicle drive system. Similarly, U.S. Pat. No. 6,952,924 describes a Rankine cycle apparatus for use in vehicle including a system to address leakage of the working medium from the system.
As another example of use of the Rankine cycle in a vehicle, United States Patent Publication Number 2004/0211180 discloses an approach for collecting waste heat from an engine through a Rankine cycle system to generate a rotational driving force.
Further, United States Patent Publication Number 2004/0184923 discloses a fluid machine operable in both a pump mode for pressurizing fluid and a motor mode for converting fluid pressure and outputting mechanical energy. A specific valve mechanism is described which enables operation of both the pump mode and the motor mode.
The inventor herein has recognized that application of a Rankine cycle system in vehicles may be limited in some applications where the working medium is not maintained at an operating temperature. Thus, the Rankine cycle system may not be fully operational during all conditions. For example, a Rankine cycle system may not produce the desired energy output during initial cold start-up of a vehicle. Time may be required for the working medium to reach a sufficient operating temperature.
Some of the issues associated with such application of a Rankine cycle system may be addressed by incorporation of a combination Rankine cycle system (or other exhaust gas heat recovery system) with a hydraulic accumulator system. In the combined system, the hydraulic accumulator system may discharge fluid power from a hydraulic accumulator when the Rankine cycle system is offline, such as the period of time when the working fluid is warming to an operational temperature.
As an example, a combination system may be used to run one or more vehicle components, such as the front end accessory drive (FEAD), including the alternator, the AC compressor, water pump, power steering pump, etc. When the engine is first started, a hydraulic accumulator system may be used to discharge fluid power to a hydraulic motor operatively coupled to a vehicle accessory. The hydraulic accumulator may provide the power to the hydraulic motor while the working medium in the Rankine cycle system is heating up. Once the working medium is at an operational temperature, the Rankine cycle system may take over driving the hydraulic pump and hydraulic motor to provide hydraulic power to run the FEAD. Upon engine shut off, the Rankine cycle system may be used to create fluid power for storage in the accumulator for use when the vehicle is restarted. By using the combination system to create FEAD drive, engine efficiency may be improved.